Method for concentrating solutions



july 25, w67 A. N. cHlRlco ETAL METHOD FOR CONCENTRATING SOLUTIONS FiledJan. 25, 1966 mwN United States Patent 3,332,470 METHOD FORCONCENTRATING SOLUTIONS Anthony N. Chirico, Naperville, and Jay D.Dockendorf, Oak Brook, Ill., assignors to Chicago Bridge & Iron Company,Oak Brook, Ill., a corporation of Illinois Filed Jan. 25, 1966, Ser. No.522,967

4 Claims. (Cl. 159-47) This invention relates to a method forconcentrating a solution and separating crystals therefrom. In its morespecific aspect, this invention relates to a method for concentrating asolution having two or more chemical solutcs, crystallizing at least onesolute, and separating the crystals from the remaining concentratedsolution. The invention is described hereinbelow in detail withreference to electrolytic caustic soda, but it should be understood thatthe invention is equally applicable to other solutions having chemicallydiderent constituents.

In the manufacture of electrolytic caustic soda, the brine charge ispartly converted through electrolysis into caustic soda. The liquorwithdrawn from the electrolytic cells typically comprises about 8 to 11%sodium hydroxide, 13 to 17% sodium chloride, and the balance water. Incommercial practice, the caustic soda is separated from the salt byconcentrating the cell liquor in a multiple effect evaporator system tocrystallize most of the salt, which is then separated from the motherliquor.

This invention has as its purpose to provide a preconcentrator for usein treating the feed liquor in combination with the multiple-effectevaporator, whereby substantial economy is realized both with respect tooperating costs and capital costs. Reference is now made herein to thespecic description and to the accompanying drawings, in which the figureis a diagrammatic representation of the present invention for carryingout our improved process.

In referring to the drawing, there is now described the concentrating ofa solution comprising caustic soda and salt as obtained from diaphragmelectrolytic cells. The liquor passes from the electrolytic cell (notshown) through a suitable supply line 10. The cell liquor is then pumpedthrough the tubes of a plurality of regenerative heaters 12, 14, 16 and18. The regenerative heaters are heated by indirect heat exchange, `andthe temperature of the cell liquor is increased gradually as it passesfrom one heater to the next. Regenerative heaters 12, 14 and 16 areheated by indirect heat exchange by the vapors flashed in the severaldash coolers 44, 42 and 40 respectively, and condensed vapors from theheat exchanger 91 of the first eiect 58 of the multiple effectevaporator system are recycled by line 99 to heat regenerative heater 18by liquid to liquid indirect heat exchange, as described hereinbelow ingreater detail. Thus, the cell liquor flows from line to heater 12, andthen through line 13 to heater 14, then via line 15 to heater 16, andthen through line 17 to heater 18. The pressure in the regenerativeheaters is sufficient to suppress boiling of the liquor and also issuicient to transport the cell liquor through the regenerative heatersto the pre-concentrator. The number of regenerative heaters employed inthe operation can vary, depending upon a number of factors, such as sizeand the operating temperature range, but the number of heaters should besuiicient to optimize heating with respect to the subsequent ash coolingstages. However, it is preferable to employ a plurality of regenerativeheaters in that, in bringing the liquor to the desired elevatedtemperature, a lesser amount of outside heat supply would be required.

From regenerative heater 18, the cell liquor, which is now at anelevated temperature, is passed via line to preheater 22. Steam issupplied to the preheater 22 from a suitable boiler (not shown) via line24 and branch line 26, to heat the liquor to a high temperature foradmission 3,332,470 Patented July 25, 1957 ice to the pre-concentrator.The steam condensate from preheater 22 is fed by line 29 to line 36which returns it to the boiler. When desired, this separate preheater,heated by a steam source, may be omitted.

After the feed liquor has been brought to the desired elevatedtemperature, and while maintained under sufcient pressure to suppressboiling, the liquor is passed from preheater 22 via line 28 to thepre-concentrator, indicated generally at 30. The pre-concentrator isdesirably a falling `film type evaporator comprising a vertical shelland tube heat exchanger 32 with tubes communicating with a vapor body 33disposed below the heat exchanger. Steam is supplied to the shell sideof the heat exchanger via line 24 and branch line 31. Because the liquoris maintained at a high temperature and under pressure, the liquorshoots rapidly through the vertical tubes and a substantial amount ofliquid is evaporated in the pre-concentrator. The vapors generated arepassed from the vapor body 33 through a vapor outlet conduit 34 for useas heating medium in the multiple eiect evaporator, describedhereinbelow, and the condensate from the preheater 22 andpreconcentrator 30 is returned to the boiler by means of a return line36.

The hot liquor, which now is relatively more concentrated because ofevaporation in the pre-concentrator, is then passed through line 38 to aplurality of ash coolers 40, 42, and 44 where liash vaporization occurs.The ternperature of the liquor and the operating pressure are reduced inthe flash coolers, each successive flash cooler being at a lowertemperature and pressure than the preceding one. The liquor enters therst flash cooler 40, which is operated at a lower pressure as comparedto the pre-concentrator and the solution adiabatically cools to theboiling temperature corresponding to the pressure existing in thevessel. The vapor generated is passed through branch steam line 46 toregenerative heater 16, thereby heating unit 16 by indirect heatexchange. The concentrated liquor is then passed through line 41 toliiash cooler 42, maintained at a lower pressure than flash cooler 40,where further evaporation occurs, and similarly, vapor generated ispassed via line 48 to regenerative heater 14 to heat that unit byindirect heat exchange. Still further evaporation of the liquor occursin ash cooler 44, maintained at a still lower pressure, to which theliquor is fed by line 43 and the steam generated is conducted throughline 50 to heat regenerative heater 12 by indirect heat exchange. As theliquor is directed through the series of flash coolers, the resultantcooling and vaporization of some of the solvent will supersaturate thesolution with respect to the sodium chloride, and sodium chloridecrystals are produced which serve as nuclei or seeds to eiect growth inthe multiple effect evaporators. This stage of the process isparticularly important in that controlled nucleation or seed generationaffects the size and habit of the crystals developed in the multipleeiiect evaporators, which is important in order to achieve optimumcrystals and to improve the plant efticiency as aected by the separationof the crystallized solute from the mother liquor. It will be understoodthat the system of regenerative heaters and Iflash coolers shown in thedrawing is purely illustrative of how savings in capital and operatingcosts can be realized, and the number, use, and arrangement of suchunits will depend upon the conditions encountered for each particularinstallation, such as steam pressure available, the degree ofconcentration of the feed liquor, the degree of concentration desired tocontrol nucleation, and the like.

The mother liquor from the last ash cooler 44 flows from line 52 to themultiple-effect evaporator system, and, as illustrated, the eiects areoperated at successively lower ltemperatures. As illustrated in thedrawing, there is the second-effect evaporator 54, the third-eiTectevaporator 56, and the first-effect evaporator 58, all of conventionalforced circulation design, and the mother liquor is conducted via line52 to the second effect of a three-effect evaporator system, but itshould be understood that the mother liquor may pass first to any of thethree evaporators. The numbering employed in referring to the particulareffect in the system is in order of decreasing operating pressure. Thefeed liquor, which contains sodium chloride crystal nuclei, is fed tothe second effect of the multi-effect evaporator system which isoperated at a subatmospheric pressure. A recirculation system isprovided for each evaporator-crystallizer effect. The recirculationsystem for the second effect 54 comprises slurry outlet line 60, arecirculation pump 62, line 61 from the pump to heat exchanger 63 and areturn slurry inlet line 64 which results in admixture of slurry withfeed liquor from line S2. The recirculation system for the first effect58 comprises slurry outlet line 90, a slurry recirculation pump 92 whichfeeds slurry by line 94 to heat exchanger 91 from which line 96 feeds itto first effect S8. The recirculation system for the third effect 56comprises outlet line 130 which feeds slurry to pump 131 which by line132 feeds the slurry to heat exchanger 133 from which line 134 feeds theslurry to the third effect 56.

Under the operating conditions employed, supersaturation of the solutionwith respect to the salt, hence further crystal growth, is produced bythe combined cooling and evaporation of the feed liquor. The seedcrystals are grown to a size and quality that facilitates theirefficient separation from the mother liquor. Vapors generated upon theevaporation of the Water from the liquor are withdrawn from the vaporzone of the crystallizer through line 68 and delivered to the nexteffect 56 in the multiple-stage system as the heating medium for thateffect by conventional heat exchange. Crystal outlet means 66 is locatednear the bottom of the crystallizer vessel 54 for removal of the crystalslurry suspended in mother liquor.

Magma comprising a slurry of product crystals of desired size suspendedin mother liquor is discharged from the crystallization section of theevaporator-crystaliizer 54 and transferred to slurry tank 70 throughline 71. The slurry tank 70 is desirably provided with a conical bottomor trim-conical bottom, and the salt crystals settle into the conicaibottom portion and are removed via line 72. The liquor is now relativelymore concentrated, and the mother liquor, having an increased content ofcaustic soda as compared to the original feed liquor from theelectrolytic cell, but still containing a substantial percentage ofsodium chloride, is decanted or Withdrawn from the slurry tank as by asuitable pump means (not shown) through line 76 to the third-effectevaporator 56 of the multiple-stage evaporator system maintained undersubatmospheric pressure. Here again, the liquor is concentrated byevaporation and, under the operating conditions, an amount of saltcrystallizes out of solution and the slurry is Withdrawn through outletmeans 78. The vapors generated pass from the vapor zone of thecrystallizer via line 80 to barometric condenser 82 which maintains avacuum in this third effect. Similar recirculation means described abovewith respect to evaporator effect 54 are provided for the third effect56. Thus, mother liquor is removed by line 130 and pump 131 and sentthrough line 132, through heat exchanger 133 and by line 134 to thirdeffect S6. Hot condensate from heat exchanger 63 is fed by line 106 toheat exchanger 133. Condensate from heat exchanger 133 is sent by line135 to storage. The magma containing crystals of desired size is passedto a second slurry tank 84 through line 73. Salt crystals which settlein the lower conical portion of the :slurry tank are withdrawn throughline 88 to the first slurry tank 70. In this manner, the salt crystalsfrom slurry tank 84 -are washed by the slurry in tank 70, therebyremoving some of the sodium hydroxide in the salt.

The relatively clear mother liquor, containing a still higher percentageof caustic soda, is withdrawn from tank 84 via line 88 to the firsteffect 58 of the multiple-stage evaporator system maintained at aboutatmospheric pressure. The liquor is withdrawn from evaporator effect 58via line 90 and is recirculated through suitable recirculating means,including pump 92 and line 94 where it admixes with feed from line 88,and is fed through heat exchanger 91 and introduced to the evaporatorthrough line 96. Vapors generated in pre-concentrator 30 are transportedfrom the v apor body 33 by line 34 to the heat exchanger 91 for use asthe heating medium for effect 58 by indirect heat exchange. The vaporcondensate from heat exchanger 91 is fed by line 99 to heater 18 fromwhich it is withdrawn by line 140 and fed to heater 16 from which it isfed by line 141 to heater 14 and from it by line 142 to heater 12. Line143 takes the condensate from heater 12 and carries it to storage.Evaporation occurs in first effect 58 thereby resulting in additionalconcentration and crystal growth. The vapors generated are withdrawnfrom the vapor space of the first effect evaporator 58 through line 98and pass to the heat exchanger 63 associated with the second effect 54to supply heat for that evaporator. Vapor condensate from heat exchanger63 is passed through line 106 to heat exchanger 133.

The solution withdrawn from the first-effect evaporator 58 is relativelyconcentrated with respect to caustic soda and now contains a very lowpercentage of sodium chloride. This concentrated solution is Withdrawnthrough branch line 102, leading from line 96, to a flash evaporator 103which may have the same general construction Ias the evaporator effectsin the multiple-stage evaporator system. The fiash evaporator 103 isoperated under subatmospheric conditions and at a temperature lower thanthe temperature in the first effect 53 whereby fiash vaporizationoccurs. The vapors generated are passed via line 104 directly to the'barometric condenser 82 and the condensate reprocessed. The liquor infiash evaporator 103 is recirculated through line 110` at the bottom ofthe vessel by pump means 112 and return line 114. The substantiallycrystal-free liquor, circulating in the recirculating means, isWithdrawn through branch line 116 to slurry tank 118. Salt crystals inthe liquor settle out under gravity and are Withdrawn from the conicalbottom portion via line 120. The salt withdrawn from slurry tank 118 iswashed by the salt and mother liquor in slurry tank 84, therebyeffecting progressive washings of the crystal sludge in each of theslurry tanks. The salt crystals Withdrawn from the bottom tank in thissuccessive arrangement is passed to a suitable centrifuge 122 forfurther separation of the salt from the mother liquor, whereby the saltmay be recovered and returned to the electrolytic cell for further.processing in the manufacture of caustic soda. The

relatively clear mother liquor, containing a very low percentage ofsodium chloride, is withdrawn from the slurry tank 118 through line 124.The liquor discharge line may pass to a surge tank 126 and then tosuitable caustic coolers 128 for further purification and recovery ofthe caustic soda product.

In the operation of the pre-concentrating and evaporating system inconcentrating electrolytic caustic soda cell liquor comprising, forexample, about 8.5% caustic soda, 16.5% sodium chloride, and the balancewater, the feed liquor from the electrolytic cell, which typically has atemperature of about 140 to 150 F., is fed from line 10 to theregenerative heaters. In these heaters, the incoming cell liquor isheated by indirect heat exchange, by the vapors generated in the ashcoolers. In this manner, the feed liquor is heated in regenerativeheater 12 to a temperature of about 200 F., and further raised to about227 F. in heater 14, and further heated in heater 16 to temperature of.257 F. The cell liquor is further elevated in temperature in heater 18to about 267 F. which was heated by indirect heat exchange .by vaporcondensate from the first effect evaporator. The cell liquor then flowsunder pressure to preheater 22 where the liquor is heated to about 329F. by means of indirect heat exchange with steam from a suitable steamsource from lines 24 and 26. The cell liquor, at this elevatedtemperature and under sufficient pressure to suppress boiling, e.g.p.s.i.g., is

introduced to the pre-concentrator 30 to effect concentration of thesolution. The pre-concentrator, for example, of the falling film type,may be maintained at a pressure of about 65 p.s.i.g., and the vaporsgenerated at 312 F. and 65 .p.s.i.g. are withdrawn through line 34 toheat exchanger 91 to heat the first effect 58 of the multiple effectevaporator system.

The liquor withdrawn from the pre-concentrator through line 38 is now ata higher concentration, e.g. 13.6% caustic soda, but the concentrationand temperature conditions are controlled so that no crystallizationoccurs until the cell liquor is treated in the ash coolers. In aconventional multiple-stage evaporator system, salt deposits graduallyaccumulate on the interior walls of the vessels. As a consequence,operations must be discontinued periodically for removing the salt. Byreason of our invention, this disadvantage is overcome in that nocrystallization occurs in the pre-concentrating stage of the operation.The liquor is cooled in the first ash cooler 40, which is operated at aslightly elevated pressure of about 35 p.s.i.g., and a temperature ofabout 291 F. The cell liquor is .progressively cooled as it passessuccessively through the flash coolers 42 and 44. Thus, fiash cooler 42may be operated at a temperature of about 261 F. and a pressure of 25p.s.i.g., and cooler 44 operated at a temperature of about 230 F. and atatmospheric pressure, As explained above, crystallization of sodiumchloride occurs in one or more of the flash coolers to generate seed forthe multiple-effect evaporators.

The liquor containing some sodium chloride crystals is then passed tothe second effect of the multiple-stage evaporator system. This effect54 is operated under vacuum and at a relatively lower temperature ofabout 192 F. The composition of the slurry withdrawn through outlet line71 comprises about 23% caustic soda, 17% sodium chloride, and thebalance water. The mother liquor passed to the third effect 56 fromslurry tank 70 is evaporated under a vacuum and at a still lowertemperature of about 147 F. The slurry Withdrawn comprises about 32%sodium hydroxide, 12% sodium chloride, and 56% water. Mother liquorpassed to the first effect 58 from slurry tank 84 is treated at anelevated temperature of about 282 F. and atmospheric pressure. Therelatively clear mother liquor having a low sodium chloride content iswithdrawn from the first effect and passed to the flash evaporator 103and then to a third slurry tank. Here, the composition of the liquorcomprises about 49.5% caustic soda and 2-3% sodium chloride. The slurryis passed to the caustic coolers and cooled to about 80 F. to causefurther crystallization of salt, thereby resulting in a solutioncontaining about 1% salt.

In addition to pre-concentrating the feed liquor to facilitateevaporation and separation of the components from the liquor, otherattendant advantages of the instant invention include substantialreductions in operating expenditures and capital costs. For example,there is substantial economy in that utilities required are reduced.These include, for example, steam, power, and water, in that the steamgenerated during the process is utilized for supplying heat at othersteps of the operation. The caustic soda feed liquor is processed at atemperature in the pre-concentrator that precludes salting out. The heattransfer surface exposed to salt formation is reduced significantly sothat much less interior surface of the equipment requires cleaning. Inview of the processing at relatively high pressures, there is areduction in volume of equipment required, and further, exoticcrystallization equipment is obviated. By reason of this invention, itis also possible to expand any existing facility of conventional designin that the invention may be incorporated with the conventionalmultiple-effect crystallizer design, thereby increasing overall plantproduction and concomitantly decreasing operating costs.

It should be understood that although the invention was describedspecifically with reference to the recovery of electrolytic causticsoda, this invention is applicable to recovery from solution of onecomponent from another by crystallizing one solute and separating thecrystals from the remaining solution. Other such solutions include, forexample, electrolytic caustic potash; chemical caustic soda containingsodium sulfate and sodium carbonate; and brines containing sodiumchloride, calcium chloride, and magnesium salts.

What is claimed is:

1. A method for recovering a first solute from liquor and recoveringliquor relatively concentrated with respect to a second solute,comprising:

(a) introducing the feed liquor to a regenerative heater to preheat saidfeed liquor and maintained under pressure sufficient to prevent boiling;

(b) introducing the resulting preheated feed liquor to apre-concentrator vessel and establishing a vapor release zone in saidpreconcentrator vessel for vaporization of solvent thereby resulting ina relatively more concentrated liquor but having substantially nocrystallization of solute;

(c) passing the liquor from the pre-concentrator to a fiash coolermaintained at a lower pressure than said pre-concentrator to effectfurther vaporization of the liquor and crystallization of solute, andemploying the vapors generated in the flash cooler as heating medium forsaid feed liquor introduced to said regenerative heater;

(d) passing the liquor, containing crystals, from the flash cooler to amultiple effect evaporator, having heat supplied thereto from the vaporgenerated in said pre-concentrator by indirect heat exchange, forfurther vaporization to effect crystallization of solute therebyresulting in a liquor relatively more concentrated than said feed liquorwith respect to a second solute; and

(e) separating a slurry of said crystals produced in said multipleeffect evaporator from said concentrated liquor.

2. A method according to claim 1 including a plurality of regenerativeheaters being interconnected for series flow, each of said regenerativeheaters being operated at successively higher temperatures; and aplurality of flash coolers being interconnected for series flow, each ofsaid flash coolers being operated at successively lower temperatures,whereby the vapors generated in each of said flash coolers are employedas heating medium for said feed liquor introduced to said regenerativeheaters in inverse order.

3. In a method for the recovery of chemically different components fromliquor wherein hot feed solution is introduced to a multiple effectevaporator whereby evaporation of the solution occurs thereby resultingin a supersaturation of the solution and crystallization, theimprovement comprising:

(a) introducing the feed liquor to a plurality of regenerative heatersbeing interconnected for series flow, each of said regenerative heatersbeing operated at successively higher temperatures and maintained undera pressure sufficient to suppress boiling;

(b) introducing the resulting preheated feed liquor to apre-concentrator vessel and establishing a vapor release zone in saidpre-concentrator vessel for vaporization of solvent thereby resulting ina relatively more concentrated liquor but having substantially nocrystallization of solute;

(c) passing the liquor from the pre-concentrator through a plurality offlash coolers interconnected for series flow, each of said flash coolersbeing operated at successively lower temperatures and maintained at alower pressure than said pre-concentrator to effect further vaporizationof the liquor and crystallization of solute, and employing the vaporsgenerated in each of said flash coolers as a heating medium for saidfeed liquor introduced to said regenerative heaters in inverse order;and

(d) passing the liquor containing crystals to a multiple eiectevaporator to effect vaporization and crystallzation of solute andseparating the crystal slurry from the resulting liquor.

4. In the recovery of chemically different components from liquor byconcentrating the liquor in a multiple effect evaporator, the method oftreating the feed liquor prior to introduction to said evaporator,comprising:

(a) introducing the feed liquor to a plurality of regenerative heatersbeing interconnected for series ow, each of said regenerative heatersbeing operated at successively higher temperatures and maintained undera pressure suflicient to suppress boiling;

(b) introducing the resulting preheated feed liquor to apre-concentrator vessel and establishing a vapor release zone in saidpre-concentrator vessel for vaporization of solvent thereby resulting ina relatively more concentrated liquor but having substantially nocrystallization of solute; and

(c) passing the liquor from the pre-concentrator through a plurality ofash coolers interconnected for series flow, each of ysaid Hash coolersbeing operated at successively lower temperatures and maintained at alower pressure than said pre-concentrator to eiect further vaporizationof the liquor and crystallization of solute, and employing the Ivaporsgenerated in each of said ash coolers as a heating medium for said feedliquor introduced to said regenerative heaters in inverse order.

References Cited UNITED' STATES PATENTS 20 NORMAN YUDKOFF, PrimaryExaminer.

WILBUR L. BASCOMB, JR., Examiner I. SOFER, Assistant Examiner.

1. A METHOD FOR RECOVERING A FIRST SOLUTE FROM LIQUOR AND RECOVERINGLIQUOR RELATIVELY CONCENTRATED WITH RESPECT TO A SECOND SOLUTECOMPRISING: (A) INTRODUCING THE FEED LIQUOR TO A REGENERATIVE HEATER TOPREHEAT SAID FEED LIQUOR AND MAINTAINED UNDER PRESSURE SUFFICIENT TOPREVENT BOILING; (B) INTRODUCING THE RESULTING PREHEATED FEED LIQUOR TOA PRECONCENTRATOR VESSEL AND ESTABLISHING A VAPOR RELEASE ZONE IN SAIDPRECONCENTRATOR VESSEL FOR VAPORIZATION OF SOLVENT THEREBY RESULTING INA RELATIVELY MORE CONCENTRATION LIQUOR BUT HAVING SUBSTANTIALLY NOCRYSTALLIZATION OF SOLUTE; (C) PASSING THE LIQUOR FROM THEPRE-CONCENTRATOR TO A FLASH COOLER MAINTAINED AT A LOWER PRESSURE THANSAID PRE-CONCENTRATOR TO EFFECT FURTHER VAPORIZATION OF THE LIQUOR ANDCRYSTALLIZATION OF SOLUTE, AND EMPLOYING THE VAPORS GENERATED IN THEFLASH COOLER AS HEATING MEDIUM FOR SAID FEED LIQUOR INTRODUCED TO SAIDREGENERATIVE HEATER; (D) PASSING THE LIQUOR, CONTAINING CRYSTALS, FROMTHE FLASH COOLER TO A MULTIPLE EFFECT EVAPORATOR, HAVING HEAT SUPPLIEDTHERETO FROM THE VAPOR GENERATED IN SAID PRE-CONCENTRATOR BY INDIRECTHEAT EXCHANGE, FOR FURTHER VAPORIZATION TO EFFECT CRYSTALLIZATION OFSOLUTE THEREBY RESULTING IN A LIQUOR RELATIVELY MORE CONCENTRATED THANSAID FEED LIQUOR WITH RESPECT TO A SECOND SOLUTE; AND (E) SEPARATING ASLURRY OF SAID CRYSTALS PRODUCED IN SAID MULTIPLE EFFECT EVAPORATOR FROMSAID CONCENTRATED LIQUOR.